This disclosure relates to systems and methods to evaluate cement behind a casing of a wellbore using a downhole tool (e.g., a neutron tool including a neutron source, such as a neutron generator).
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as an admission of any kind.
A wellbore drilled into a geological formation may be targeted to produce oil and/or gas from certain zones of the geological formation. To prevent zones from interacting with one another via the wellbore and to prevent fluids from undesired zones entering the wellbore, the wellbore may be completed by placing a cylindrical casing into the wellbore and cementing the annulus between the casing and the wall of the wellbore. During cementing, cement may be injected into the annulus formed between the cylindrical casing and the geological formation. When the cement properly sets, fluids from one zone of the geological formation may not be able to pass through the wellbore to interact with another zone. This desirable condition is referred to as “zonal isolation.” Yet, well completions may not go as planned. For example, the cement may not set as planned and/or the quality of the cement may be less than expected. In other cases, the cement may unexpectedly fail to set above a certain depth due to natural fissures in the formation.
Acoustic downhole tools are often used to identify whether the cement has been properly installed. For example, an acoustic downhole tool may be placed into the wellbore. The acoustic downhole tool may emit an acoustic signal toward the casing. The acoustic signal impacts the casing, causing the casing to produce an acoustic response signal, which varies depending on the acoustic impedance of the material on the other side of the casing. The acoustic response signal may indicate that a solid is on the other side of the casing when relatively heavy cement is properly installed behind the casing. Increasingly, however, lighter rather than heavier cements are being used to complete wells. These may be difficult to detect using some acoustic downhole tools. Moreover, in addition to acoustic downhole tools, other well logging tools may be used to assess well properties.